What increases the thermal efficiency of a piston engine?

Increasing the compression ratio is a key factor in enhancing the thermal efficiency of a piston engine. The thermal efficiency of an engine is influenced heavily by how much work is extracted from the heat energy provided by fuel combustion. When the compression ratio is increased, the engine compresses the air-fuel mixture more before ignition. This results in a higher temperature and pressure at the point of combustion.

A higher compression ratio allows for a more complete and efficient burn of the fuel, as the increased pressure leads to a more effective combustion process. This means that the engine can extract more useful work from the same amount of fuel, thus increasing thermal efficiency. Moreover, higher compression ratios facilitate better thermal management and can lead to improved fuel economy and performance.

While other factors like engine speed (R.P.M.), combustion chamber volume, and ignition timing can influence an engine's performance, they do not directly enhance thermal efficiency in the same fundamental way that raising the compression ratio does. For instance, simply increasing R.P.M. can lead to more power generation, but this does not necessarily improve thermal efficiency, as it may lead to higher energy losses due to friction and heat.